1. Field of the Invention
The present invention relates to an optical bidirectional transceiver for transmitting and receiving an optical signal bidirectionally through a single optical fiber, and more particularly, to an optical alignment method of reducing light input reflected from a certain end point on a transmission line and entering a laser diode to reduce a Relative intensity noise (RIN) generated by the reflected light and to secure optical coupling efficiency at the same time, and an optical alignment apparatus.
2. Description of the Related Art
RIN occurring in an optical transceiver module increases in proportion to the quantity of reflected light input that is reflected from a certain point of an optical transmission line and enters a laser diode. Up to now, the RIN is reduced, as disclosed in U.S. Pat. No. 4,926,430, by an optical isolator provided between the laser diode and the optical fiber to intercept the reflected light input from the optical fiber to the laser diode.
In a case of using the optical isolator, when the quantity of the reflected light entering the laser diode is reduced, RIN is reduced and quality of a signal is improved. However, since the optical isolator is expensive, manufacturing costs of the optical transceiver module increase. Moreover, since the optical isolator is implemented in the form of bulk type, it is difficult to minimize the size of the optical transceiver module.
Thus, it is more difficult to apply the optical isolator to an optical transceiver module for a subscriber network application requiring low costs and small size.
In a case of a bidirectional optical transceiver module for a subscriber network application, the optical alignment is attempted between the optical fiber and the laser diode without the optical isolator. In this case, the optical fiber is aligned with the laser diode to detected the maximum light output (This is a technology commonly accepted in the passive alignment and the active alignment.), and after the optical alignment, the optical fiber is fixed at the aligned position by laser welding, soldering, epoxy, or the like depending on the configuration properties of the module.
FIG. 1 is a block diagram illustrating a conventional optical alignment apparatus for aligning optical axes to an optical subassembly 10 and an optical fiber 20. The conventional optical alignment apparatus 30, in order to focus a light outputted from a focusing lens 12 of the optical subassembly 10 to an input end of the optical fiber 20, measures light power outputted from the optical fiber 20 with an first optical power meter 31 and feeds a measured light output signal to a controller 32. The controller 32 controls an optical aligning device 33 to adjust the position of the optical fiber 20 such that the light output power measured by the optical power meter 31 gets maximal.
As such, the conventional optical alignment apparatus 30 determines the alignment position of the optical fiber 20 by considering only the light output power 11b such that the light output is maximal.
Thus, in the optical transceiver comprising the optical isolator, since the incident quantity of external reflection is small, the optical alignment is carried out only with respect to the maximum light output as described above to achieve the maximum optical coupling efficiency. However, in a configuration coupling the optical fiber 20 with the laser diode 11 without isolator, since a great deal of reflected light enters the laser diode 11, the quality of a signal is deteriorated when the optical alignment is carried out only by taking the maximum output into consideration without the above-mentioned RIN.
In other words, in a case of fixing an optical system by carrying out the optical alignment with respect to the position of the maximum light output, since the quantity of the external reflection entering the laser diode 11 is also maximized, the RIN is also maximized like the experimental result illustrated in FIG. 2.